High impact strength, high glass transition temperature (Tg), and resistance to thermal degradation, are among the most desirable properties of commercially successful high performance resins. Polymers of arylene ether ketones and arylene ether sulfones, referred to as polyarylene polyethers and polyarylene polythioethers, together referred to herein as polyarylene poly(thio)ethers (PAPE) such as those disclosed in U.S. Pat. No. 4,562,243, rank high among polymers which have such desirable properties, particularly excellent thermal properties and impact strength. The emphasis has been to produce other aromatic polyethers which have even better properties, and might be even more economical to manufacture. Of particular interest are thermoplastics of this kind having a Tg high enough for advanced aerospace applications, exceeding that obtainable with the aforesaid PAPE polymers. High Tg PAPE polymers would result from appropriate monomers containing fused aromatic rings, particularly if polymerization occurred through coupling of such rings. This invention describes the preparation of such polymers.
The polymers of this invention are prepared by the well known Scholl reaction which is generally applied to the dehydrogenative condensation of aromatic compounds to yield dimers, or other relatively small molecules. The dehydrogenative condensations (also referred to as "oxidative polymerization") can take place in either an inter- or an intramolecular way. Intermolecular Scholl reactions are numerous and include such reactions as the formation of biphenyl from benzene: ##STR1##
Both inter- and intramolecular dehydrogenation of aromatic nuclei proceed in the presence of oxidizing agents, the latter resulting in the formation of a condensed ring system. These condensations occur with oxidizing agents that can include certain transition metal salts, e.g. FeCl.sub.3 or CuCl.sub.2, or Lewis acid catalysts in oxidizing media, e.g. nitrobenzene.
The elimination of two aryl-bound hydrogens accompanied by the formation of an aryl-aryl bond under the influence of Friedel-Crafts catalysts is referred to as the Scholl reaction. (see "Friedel-Crafts and Related Reactions" by George Olah, vol II, Part 2, Chapter XXIII titled "Dehydrogenation Condensation of Aromatics (Scholl and Related Reactions)" by A. T. Balaban and C. D. Nenitzescu, Interscience Publishers 1964.
In an article titled "Studies of the Scholl Reaction: Oxidative Dehydrogenation involving 1-Ethoxynaphthalene and Related Compounds" by G. A. Clowes, J.Chem.Soc., C, 2519 (1968) the author reportedly obtained a very low molecular weight (mol wt) poly(binaphthylene oxide) from the oxidation of 1,1'-binaphthyl ether. In light of his results, it is perhaps to be expected that Clowes did not suggest the application of the Scholl reaction for the preparation of relatively high mol wt polymers by the polymerization of monomers containing aromatic rings.
But some time later, purportedly "high" mol wt linear poly(dinaphthyl alkylene ether) polymers of di(1-naphthoxy) alkanes were obtained by Feasey et al as disclosed in U.S. Pat. No. 3,810,870 and in an article titled "Preparation of poly(dinaphthyl alkylene ethers) by oxidative polyarylation and their crystallization behaviour". These polymers were derived, like that of Clowes, from either bis-(1-naphthyl)ether having the structure: ##STR2## or from a di-1-naphthoxy alkane having the structure: ##STR3## wherein m is an integer in the range from 1 to about 20, so that the repeating unit would have the following structure: ##STR4## wherein E is a direct link or a group having the formula --X--O-- where X is a bivalent non-polar residue.
Since Feasey et al teach away from the use of polar residues, one would expect that the particular non-polar linking of the naphthoxy moieties was essential to provide the purportedly high mol wt polymers which Clowes failed to produce without it. Note however, that there is no data, other than the reduced viscosity in the Table in col 4, to provide evidence of mol wt, and as one skilled in the art will appreciate, the reduced viscosity alone cannot define the mol wt of the polymer. Though a wide range of non-polar bivalent aromatic or aliphatic residues containing up to 20 carbon atoms, is listed, there is now reason to believe that Feasey's choice of non-polar residue may not have been as effective as it was hoped. The results of polymerization with a non-polar 2,2'-biphenylene residue specifically suggested by Feasey et al, which results are presented hereinbelow, indicate an unsatisfactory polymerization. It is one thing to expect the Scholl reaction to proceed without interfering chemical events when only two, three, or less than about 10 molecules are to be successively dehydrogenated thus forming a macromolecule; it is a wholly different matter to expect the reaction to so proceed unerringly over the course of condensing a hundred or more molecules into a long chain.
A recent review article by W. Koch, W. Risse and W. Heitz, Makromol Chem 184 779 (1983) surveys the polycondensation reactions in which radical-ions are considered responsible for chain propagation in polycondensation reactions, e.g. the synthesis of poly(2,6-dimethyl-1,4-phenylene oxide), poly(1,4-phenylene oxide), poly(1,4-phenylene sulfide), and poly(1,4-phenylene). In such reactions in which the mechanism is described by Heitz as a "reactive intermediate condensation", charge at the cationic end group can be delocalized through connected aromatic segments. To the extent that such delocalization applies to the polymerization of di(1-naphthoxy) monomers, then polar, electron withdrawing connecting groups could increase the oxidation potential, making oxidative polymer ization more difficult.
The formation of polymers having a large fraction with mol wt greater than about 10,000 is made even more difficult because the chains tend to undergo a ring closure, resulting in cyclic oligomers. Further, we have found that successful polymerizations can be temperature sensitive, resulting, for one specific example, in crosslinking at temperatures exceeding about 80.degree. C. The formation of cyclic polymers and crosslinking can be sensitive to the particular character of the linking group, but the extent of such sensitivity for each polymer can only be determined by actual testing.
Particularly since the known requirement for the linking group was that it include a non-polar residue, there was no basis for reaching an informed conclusion that a linking residue containing a polar group might lend itself to a successful Scholl polymerization. There was no reason to explore the possibility that a di(4-naphthoxyphenyl) repeating unit linked with a polar residue, or a linking group containing a polar residue, might yield a polymer with desirable properties.